Sub-patch optimization techniques for use in a hair/fur pipeline

ABSTRACT

A surface definition module of a hair/fur pipeline may be used to define a surface and an optimization module may be used to: create a bounding box for a set of hairs; determine whether the bounding box is visible; and if the bounding box is visible, hair associated with the visible bounding box is rendered upon the surface.

This application is a continuation-in-part of application Ser. No.11/345,355 filed on Feb. 01, 2006, which is a continuation ofapplication Ser. No. 10/052,068 filed on Jan. 16, 2002, now issued asU.S. Pat. No. 7,050,062 which is a continuation of application Ser. No.09/370,104 now issued as U.S. Pat. No. 6,952,218. This application alsoclaims priority to provisional patent application 60/833,113 filed Jul.24, 2006 and provisional patent application 60/886,286 filed Jan. 23,2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the digital creation of fur. More particularly,the present invention relates to the digital creation of realisticclose-up and distant looks of fur coats on animal models.

2. Related Art

One of the many challenges in modeling, animating and renderingbelievable mammals in computer graphics has been to producerealistic-looking fur. A real fur coat is made up of hundreds ofthousands of individual, cylindrical hairs covering the skin, andfulfills vital functions such as protection against cold and predators.Between animals as well as across the body of individual animals, thelook and structure of these hairs vary greatly with respect to length,thickness, shape, color, orientation and under/overcoat composition. Inaddition, fur is not static, but moves and breaks up as a result of themotion of the underlying skin and muscles, and also due to externalinfluences, such as wind and water.

Some prior computer graphics techniques used for fur creation haveachieved convincing looks of smooth fur; however, these techniques donot take into account that real fur often breaks up at certain areas ofthe body, such as around the neck. In addition, the prior methods do notaccount for hairs of wet fur clump together resulting in a significantlydifferent appearance compared to dry fur. Also, the process ofsimulating hair as it is getting increasingly wet when sprinkled on bywater has not yet been addressed.

SUMMARY OF THE INVENTION

The system and method of the present invention provides a flexibletechnique for the digital representation and generation of realistic furcoats on geometric models of surfaces, such as animals. In oneembodiment, an innovative technique for placement, adjustment andcombing of fur on surfaces is provided. In one embodiment, thecontinuity of fur across surface patch boundaries is maintained. Inaddition, in one embodiment, an innovative method to simulate wet fur isprovided. In this method static clumping and animated clumping may beapplied to regions on the surfaces. In one embodiment, a method for thesymmetric and one-sided breaking of hairs along fur-tracks on surfacesis provided. The above processes can be iteratively applied in order togenerate layers of fur, such as an undercoat and an overcoat.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention will beapparent from the following detailed description in which:

FIGS. 1 a and 1 b are simplified block diagrams of embodiments ofsystems that operate in accordance with the teachings of the presentinvention.

FIG. 2 is a flow diagram of one embodiment of a process for thegeneration of fur in accordance with the teachings of the presentinvention.

FIG. 3 a is an illustration of a set of parametric surfaces defining theskin of a three-dimensional animal model.

FIG. 3 b is a simplified flow diagram illustrating one embodiment ofstatic and animated combing processes.

FIGS. 3 c and 3 d are examples that illustrate one embodiment of thecombing processes.

FIG. 4 is a flow chart illustrating one embodiment of a process foradjusting control hairs for removing visual discontinuities at surfaceboundaries.

FIG. 5 is a flow chart illustrating one embodiment of a process forplacing hairs.

FIG. 6 illustrates one example of subpatches defined on the surface.

FIG. 7 a illustrates an example of control vertices of one control hair.

FIG. 7 b illustrates an example for calculating control hair weights.

FIG. 7 c illustrates an example an interpolation process used tocalculate orientations of final hairs in accordance with the teachingsof one embodiment of the present invention.

FIG. 7 d is a simplified flow diagram of one embodiment for thecalculation of the orientation of final hairs.

FIG. 8 is a flow chart illustrating one embodiment of a process toperform static clumping.

FIG. 9 illustrates examples of different clump-percent and clump-ratevalues.

FIG. 10 a shows a rendered frame of a combed fur coat and FIGS. 10 b, 10c and 10 d show snapshots of one embodiment of an animated dry-to-wetfur sequence.

FIG. 11 is a flow chart illustrating one embodiment of a process foranimated area clumping.

FIG. 12 a is a flow chart illustrating one embodiment of a process forhair breaking.

FIG. 12 b illustrates examples of symmetric and one-sided breaking ofhairs.

FIGS. 12 c, 12 d, 12 e, and 12 f illustrate examples of breakingeffects.

FIGS. 13 a, 13 b, and 13 c illustrate the visual effects of undercoatand overcoat.

FIG. 14 is a flow chart illustrating one embodiment of a shadingprocess.

FIG. 15 is a block diagram illustrating an embodiment of a hair/furpipeline, similar to the pipeline of FIG. 1 b, but that includesadditional and differing functionality.

FIGS. 16 a and 16 b are diagrams that illustrate the clumping of controlhairs with possible variations along the control hairs.

FIG. 17 is a flow diagram illustrating a process 1700 to implement afill-volume function, according to one embodiment of the invention.

FIGS. 18 a-c are diagrams illustrating the generation of braid shapesdefining surfaces and associated volumes (FIG. 18 a), the filling ofthese volumes with randomly placed control hairs (FIG. 18 b), and theinterpolation of final hair strands from the control hairs (FIG. 18 c).

FIGS. 19 a-c are diagrams showing side views of a deformed surface withidentical control hairs that illustrate different types of interpolationtechniques, respectively.

FIGS. 20 a-20 c are diagrams illustrating wave, weave, and wind effects.

FIG. 21 is a flow diagram illustrating a process to implement geometricinstancing, according to one embodiment of the invention.

FIGS. 22 a and 22 b are diagrams illustrating an example of geometricinstancing.

FIG. 23 is a diagram illustrating a simple graph of a static nodeconnected to an animation node and a finished control node to provide ahair motion compositing system, according to one embodiment of theinvention.

FIG. 24 is a diagram illustrating a process of utilizing a blend node.

FIGS. 25 a and 25 b are diagrams illustrating rotational blending andpositional blending, respectively.

FIG. 26 is a diagram illustrating a blend ball.

FIG. 27 illustrates a dynamic node graph including a dynamic solvernode.

FIGS. 28 a and 28 b are diagrams illustrating the use of volume node.

FIG. 29 is a flow diagram illustrating a process super hair nodeprocessing is illustrated.

FIGS. 30 a and 30 b are diagrams illustrating super hair operations inlocal-space and world space, respectively.

FIG. 31 is a diagram illustrating a blend ball having both an innersphere and an outer sphere.

FIG. 32 is a diagram illustrating a cascading node graph to blendbetween various simulation caches.

FIG. 33 is a diagram illustrating techniques to implement view-dependentscreen-space optimization.

FIG. 34 is a diagram illustrating a hair follicle root position and ahair that is transformed to a normalized device coordinate (NDC) system.

FIG. 35 is a diagram illustrating the distance traveled by the root of aproxy hair from a first frame to a second frame in NDC space.

FIG. 36 is a table illustrating a side-by-side comparison ofnon-optimized values versus optimized values in terms of hair count,time, and memory using the screen-space size metric.

FIG. 37 is a table illustrating another comparison utilizing thescreen-space speed method.

FIG. 38 is a table illustrating non-optimized and optimized hair count,time, and memory values.

FIG. 39 is a flow diagram illustrating a process to implement hairsub-patch optimization.

FIG. 40 is a diagram illustrating a simplified example of the use ofhair sub-patch optimization.

FIG. 41 is a diagram showing a grassy landscape that was modeled, inwhich, the hairs were geometrically instanced with grass and/or trees,and that utilizes sub-patch optimization techniques.

FIG. 42 is a diagram illustrating an example of a cache state file.

FIG. 43 is a flow diagram illustrating a process to implementhair-caching.

FIG. 44 is a table showing time-savings that can accomplished usinghair-caching to render a fully-furred character.

DETAILED DESCRIPTION

The method and apparatus provides an innovative technique for thedigital generation of fur on surfaces, such as on a computer generatedanimal. FIG. 1 a is a simplified block diagram of one embodiment thatoperates in accordance with the teachings of the present invention.Computer system 10 includes central processing unit (CPU) 15, memory 25,Input/Output 20, which may be coupled to a storage device such as a diskdrive or other device. The system may also include a keyboard 40 orother user input device as well as a display 35 that may be used todisplay user interface and the final rendering of the fur in accordancewith the teachings of the present invention.

In one embodiment, memory 25 stores instructions which when executed bythe CPU 15 performs the processes described herein. Alternately, theinstructions may be received via storage 30 or other input such a userinput 40. The processes described herein may be executed via software bya system such as system 10 or hardware, or a combination of bothhardware and software.

An alternate embodiment is illustrated in FIG. 1 b. Input is received bysurface definition module 50 that defines a surface which, as will beexplained below, defines surfaces and control hairs of the object to berendered. Module 55 adjusts the control hairs to provide suchfunctionality as combing and seamless hairs across surface boundaries.The interpolation module 60 interpolates across the surfaces using thecontrol hairs. Hair clumping and breaking module 65 enhances therealistic visualization of the object by providing for clumping andbreaking of hairs. Rendering module 70 renders the hairs and providesshading, black lighting and shadowing effects to the hairs, and module75 displays the final output of the object with the hair surfaces

FIG. 2 is a flow diagram of the steps involved in generating fur coat inaccordance with the teachings of the present invention. At step 200, thegeometry of the surfaces that contain the hair is defined. In oneembodiment, a three-dimensional geometry may be used to model the skin,for example an animal skin, on which the fur coat is later generated. Asillustrated in FIG. 3 a, the geometry is usually defined as a connectedset of parametric surfaces often referred to as surface patches. Thepatches can be generated a number of ways known to one skilled in theart. In one embodiment, NURBS surface patches are used.

Referring back to FIG. 2, at step 210, control hairs are placed ontothese surface patches, whereby each control hair is modeled as aparametric curve, e.g., a NURBS curve, defined by a user-specifiednumber of control vertices. As will be discussed below, a global densityvalue for the hair is given by the user to determine the number ofactual hairs and their positions on the surface patches. Each hair alsohas a number of attributes such as length, width, waviness, opacity, andby default points in the direction of the surface normal at its positionon the surface.

In the present embodiment, a number of operations are performed oncontrol hairs and the final hairs are generated based upon the controlhairs and other information. However, it should be realized that thesesteps, such as combing and the like described herein may be performed onthe final hairs instead of the control hairs.

A number of different approaches may be taken to generate the controlhairs. One simple algorithm places equally spaced x hairs in the udirection and y hairs in the v direction (where x and y are specified bythe user) of each NURBS patch. Alternately, the x and y hairs are placedequally by arc-length. This will result in a more uniform distributionacross the patch. However, it does not achieve a balanced distributionof control hairs across patches of different sizes; x and y hairs areplaced on all selected patches, whether large or small. Therefore, in analternate embodiment, the generation of control hairs takes into accountthe area of a NURBS patch to determine x and y for each patch. In oneembodiment, the user specifies z hairs per unit area. In addition in oneembodiment, control hairs can also be placed individually or alongcurves-on surfaces for finer control. For example, extra control hairsalong the sharp rim of the ears of an animal can be generated to ensureproper alignment of the fur eventually generated.

Referring back to FIG. 2, once the control hairs are generated, step210, the control hairs are adjusted at the surface boundaries. Sincecontrol hairs are placed in each surface patch, control hairs that lieon the boundary of a surface patch might not line up with control hairson a neighboring surface patch; this can lead to visible discontinuitiesof the hairs along the surface boundaries. To address the potentialproblem, the control hairs on surface boundaries are adjusted. Oneembodiment of the process for adjusting the control hairs is illustratedby the flow chart of FIG. 4.

At step 400, seams are constructed between adjacent surfaces. Each seamidentifies adjacent surfaces along a corresponding boundary (forexample, an entire edge, T-junctions, or corners) of a surface patch. Atstep 405, for each surface patch, the boundaries are traversed, step410. Each control hair is examined, step 412. At step 415, if a boundaryhair is found, at step 420, the neighboring patches, as identified by acorresponding seam, are checked to see if there is a corresponding hairon the neighboring patch. In one embodiment, a hair is corresponding ifit is within a small predetermined distance from the boundary hair. Thedistance may be specified in parametric u, v, or absolute space. In oneembodiment, the predetermined distance may be a relatively smalldistance such that the hairs visually appear co-located.

If there is a corresponding control hair, the boundary hair and thecorresponding hair are aligned, step 425, by modifying the location andorientation of one or both of the control hairs to a common location andorientation, respectively. In one embodiment, the corresponding hair ofan adjacent surface patch is snapped to the location of the boundaryhair along the boundary. In one embodiment, if the neighboring surfacepatch does not have a corresponding hair, then one is inserted andaligned on the neighboring patch, step 445. The process continues foreach boundary hair along each boundary in each surface patch, steps 430,435 and 440 until all boundary hairs are aligned.

Referring back to FIG. 2, after step 215, in one embodiment, the controlhairs have been placed on the surfaces defining an animal or otherobject model and the control hairs point along a surface normal at theirpositions on the surface.

At step 220, the hairs are combed to achieve desired, groomed, dry furlooks. A number of different combing processes may be used. In thepresent embodiment, however, static and animated combing processes areapplied to the control hairs. The combination of static and animatedcombing provides a low computation cost but effective visual effect. Inalternate embodiments, static or animated combing alone may be used andgenerate beneficial visual results. The combing process may be used onthe same control-hairs for different shots, to provide, for example, agroomed look versus a slightly messy look of the fur.

One embodiment will be described with reference to FIG. 3 b. In oneembodiment, static combing is applied if the hairs do not “actively”move during an animation of the object, e.g., the animal. It should benoted that since each hair is expressed in a local coordinate systemdefined by the surface normal, du and dv at the root of the hair,statically combed hairs will “passively” move as the underlying surfaceis deformed or animated. Combing is achieved by specification of combingdirection curves, degree of bending and curvature of the hair, as wellas with a fall-off for each combing direction curve.

At step 325, one or more combing direction curves are created. Thesecurves indicate the direction applicable control hairs are to be combed.An example is illustrated by FIGS. 3 c and 3 d. FIG. 3 c shows a numberof uncombed control hairs. FIG. 3 d illustrates an exemplary combingdirection curve 365 and its direction. The combed hairs as alsoillustrated in FIG. 3 d.

FIG. 3 d illustrates one combing direction curve. However, it is commonto implement a number of different curves, each curve corresponding to adifferent area of the surface. Thus, at step 330, for each curve, one ormore control hairs are assigned such that the assigned hairs are combedin accordance with the corresponding combing direction curve.

In addition, at step 335, for each curve, bend, curvature and falloutparameters are defined. The bend parameter defines how close the controlhair is to the surface. The curvature parameter indicates the shape ofthe hair. For example, a curvature value of zero may indicate the hairis to be straight up and a maximum value (e.g., 1) may indicate the hairis bent on a tight arc from root to tip.

The fallout value indicates a region beyond which the combing directioncurve decreases its influence the further away the control hair is fromthe curve. In some embodiments, the fallout region is specified to coverrelatively large areas so all control hairs are equally affected andfallout does not occur. In other embodiments it is desirable to decreasethe combing effect, the further the distance between the control hairand the combing direction curve.

At step 340, each control hair is processed according to the combingdirection curve it is assigned to, and the bend, curvature and falloutparameters. The result of the process is illustrated by FIG. 3 d inwhich the hairs are combed in the direction of the combing directioncurve 365. In addition, the hairs are curved according to the bend andcurvature parameters defined. In the present example the falloutparameter defines the entire surface such all hairs are equally affectedand fallout is not present.

As noted above, animated combining may also be applied, step 345. Keyframing, known in the art, is used to interpolate between combingchanges specified at certain frames to proving smooth transitionsbetween changes. Thus for example, bend curvature and fallout parametersmay be specified to change at certain frames. The key framing processexecution then transitions during the frames between the specified framechanges. This technique can be used to simulate a variety of conditionswhich affect the look of the hair, such as wind. Thus, the hairs can beanimated by key framing the parameters and executing the combingcalculations at each frame during playback.

The combing process may also include a simple hair/surface collisionmodel process in which hairs that intersect with the underlying surfacedue to the combing process are pushed back up to be above the surface.Hairs may be rotated to intersect the underlying surface due to, forexample, by setting the bend parameter to a large value.

The process includes an iterative algorithm that determines hair/surfaceintersections. For example, the process performs a line segmentintersection check of successive control vertices of a curve (e.g., theNURBS curve) defining a control hair with the surface. If a controlvertex c goes below the surface, the hair is rotated back towards thesurface normal from the previous non-intersecting vertex just enough forc to clear the surface. The amount of rotation is large enough to causethe hair to rotate back up above the surface by a small amount specifiedby the application. Thus the vertices of the vector affected by thecombing are rotated back towards the surface normal so that the vectoris above the surface.

In an alternate embodiment, the combing may be animated by turning eachcontrol vertex of a control hair into a particle, and applying dynamiceffects like gravity and external forces. Software, such as Maya,available by Alias|Wavefront, a division of Silicon Graphics, Inc.,Toronto Canada, may be used to perform this function.

Once the control hairs are identified and processed (e.g., adjusted,combed), the final hairs for each patch are generated from the controlhairs, step 223, FIG. 2. As noted above, in one embodiment the controlhairs are first adjusted along surface boundaries. In an alternateembodiment, combing may be applied to control hairs alone or inconjunction with application of the surface boundary adjustment process.

One exemplary process for the placement of hairs on patches isillustrated by the flow chart of FIG. 5. In this embodiment, final hairsare generated from control hairs in two set steps. First, the statichair features are calculated, e.g., the placement (the u, v position) ofthe final hairs. This step may be performed once. The second set ofsteps may be performed for each frame in an animation and provide framedependent hair features.

At step 505, subpatches are identified on a surface of the object onwhich the hairs are to be generated. FIG. 6 illustrates one embodimentfor placing the final hairs on the surfaces defining the underlyingskin. In the present embodiment, the root positions for each final hairare determined in terms of

(u, v) parametric values of the surface. They are computed from anoverall (global to all surfaces) density input value dmax (number ofhairs per square unit area), and a set of normalized local densityvalues (values range from 0 to 1; default value may be 1) per surfacepatch, arranged at a variable resolution grid of equally spaced pointson the surface (for example, 128×128 points).

In one embodiment, the process attempts to make the number of hairsindependent of the resolution of this grid and independent of thesurface patch size to provide seamless density across surfaces ofdifferent scale. For purposes of discussion, it is assumed that thespecified input density value (dmax) is 10 hairs/unit square area, andthe local density values are arranged at equally spaced points on thesurface as shown in FIG. 6 (e.g., 0.4, 0.5, 0.6, 0.6 hairs,respectively). These points define the subpatches of the surface to beprocessed, step 505, FIG. 5. As these equally spaced points aretraversed, step 510, the area in (u, v) space between the neighboringpoints is approximated by the area defined by two polygons, moreparticularly, triangles (a1 and a2), and the number of hairs/square unitarea for each triangle hair unit is averaged from the values at itsvertices step 520. In one embodiment, this is determined according tothe following: HairUnit=dmax*Vavg, where dmax represents the specifiedinput density value and Vavg represents the average local density valuefor each triangle determined from values at its vertices. For theexample defined, this results in 10*(0.4+0.5+0.6)/3=5 and10*(0.4+0.6+0.6)/3=5.333 hairs/square unit area for the top left andbottom right triangle, respectively.

At step 525, the total number of hairs to place on the current subpatchis determined from the actual approximated area of the subpatch (a1 anda2) and the number of hairs per unit area. In one embodiment, the totalnumber of hairs per unit area is determined according to the following:HairTotal=A*HairUnit, where A represents the actual approximated area ofthe subpatch. For example, if a value of 0.4 applies to area a1 and 0.3applies to area a2, as assumed for purposes of discussion,0.4*5+0.3*5.333=3.5999 is the total number of hairs to place in subpatchdefined by (ui, vi), (ui,vi+1), (ui+1,vi+1), (ui+1,vi).

At step 530, the final hairs are placed. Since it is preferable not toplace fractional hairs, either 3 or 4 hairs are placed depending onwhether a uniformly generated random number in [0,1] is bigger orsmaller than the fractional part (0.5999). The 3 or 4 control hairs arerandomly placed in u [ui, ui+1] and randomly in v [vi, vi+1]. Theprocess then proceeds back to step 515 to the subpatch defined by thenext four equally spaced points.

Each final hair contains a number of control vertices. The root position(first control vertex) of each control hair is specified in terms of a(u,v) value of the underlying surface. The remaining control vertices ofeach hair are defined in a known local coordinate system with originsspecified at the hair root position, and axes in the direction of thesurface normal, du, dv. In one embodiment, each hair is oriented alongthe surface normal and the coordinates of the control vertices aregenerated by subdividing the length of each hair into n−1 equal parts,where n is the number of control vertices/hair. One example isillustrated in FIG. 7 a, where a hair 725 is defined on surface 730 withn=4. The root is vertex 720 and the remaining vertices are 705, 710 and715.

Once the root position is calculated the enclosing control hairs (in oneembodiment three) for each final hair are determined. In one embodiment,a 2-dimensional Delaunay triangulation (know in the art and thereforenot further discussed herein) is constructed of the (u,v) positions ofthe control hairs for each surface patch. This triangulation was chosenbecause it creates “well-proportioned” triangles, by minimizing thecircumcircle and maximizing the minimal angles of the triangles. Oncethe Delaunay triangulation is constructed, it is determined whichtriangle each final hair falls into. The indices of the three controlhairs which form the particular triangle are assigned to the hair thatfalls into that triangle.

The weights (w1, w2, w3) which each of the three control hairs (c1, c2,c3) has on the final hair (h) are then calculated. This may be doneusing barycentric coordinates (know in the art and not further discussedhere), and is illustrated in FIG. 7 b, where “A” represents the area oftriangle 726 (c1, c2, c3). These weights are used for interpolating thefinal hairs from the control hairs as explained below.

The above information of each final hair (i.e., the (u, v) position, the3 enclosing control hairs, and the weights of each control hair) may begenerated only once for an object in animation. This information isreferred to herein as the static information. In contrast, thecalculation of the orientation of each final hair may be done at eachframe of an animation. This orientation is determined from theorientation of the control hairs and their corresponding weights by aninterpolation process as explained with reference to FIGS. 7 c and 7 d.

For each final hair (h) at step 756, the corresponding three controlhairs (c1, c2, c3) are converted into a surface patch space (in oneembodiment, the patch coordinate system) utilized. At step 758, controlhair vectors, (e.g., v11, v12, v13), between control vertices arecalculated (e.g., 782, 783). A variety of techniques may be used tocalculate the control hair vectors; in one embodiment, the vectors areequally distributed between control vertices. The control hair vectorsare then normalized (e.g., nv11, nv12, nv13). At step 760 thecorresponding control hair vectors of the three control hairs areinterpolated and multiplied by the three determined weights that werecalculated for the final hair. In one embodiment, for example, onecontrol hair vector is determined according to the following equation:iv1=nv11*w1+nv21*w2+nv31*w3;

wherein iv1 represents the interpolated, weighted control hair Vectorrepresentation of the final hair, nv11, nv21 and nv31 representnormalized control vectors and w1, w2 and w3 represent the correspondingweights for the normalized control vectors. At step 762, the resultingvectors are scaled to a final hair length (siv1, siv2, siv3). Once thescaled vectors are determined, at step 764 the control vertices (786,788, 790, 792) of the final hair are calculated from the scaled vectors.

As illustrated in FIG. 2, steps 225, 230, 235 and 245 may be optionaland may not be necessary to produce a groomed, dry fur coat. Steps 225,and 230 are applied for the generation of wet fur. Clumping of hairs canoccur when the fur gets wet due to the surface tension or cohesion ofwater. The effect is that the tips of neighboring hairs (a bunch ofhairs) tend to gravitate towards the same point, creating a kind ofcone-shaped “super-hair”, or circular clump. As will be described below,step 225 is executed for static area clumping that generates hair clumpsin fixed predefined areas. Step 230 is executed for animated areaclumping, that is, when clumping areas move on the model, for example,to simulate spouts of water or raindrops hitting the fur and making itincreasingly wet. In both cases, parameters which can be animated areprovided to achieve various degrees of dry-to-wet fur looks. Step 235 isapplied to generate dry fur clumping or breaking.

According to a particular application all or some of the steps 225, 230and 235 may be executed. In addition, the steps 225, 230 and 235 may beprioritized according to an application such that a hair adjusted in ahigher priority step is not adjusted in the other steps. Alternately,the effects may be cumulative or selectively cumulative depending upon aparticular application.

At step 225 static area clumping is performed. One embodiment of thisprocess is described with reference to FIG. 8. For purposes ofdiscussion, the center hair of each clump is referred to as theclump-center hair, and all the other member hairs of that clump, whichare attracted to this clump center hair, are referred to herein as clumphairs.

In one embodiment there are four clumping input parameters:clump-density, clump-size, clump-percent and clump-rate. Similar to thehair-density parameter, clump-density specifies how many clumps shouldbe generated per square area. The process described herein translatesthe clump density into an actual number of clumps defined byclump-center hairs, the number of clump center hairs depending on thesize of each surface patch. As a result, some of the existing hairs areturned into clump-center hairs.

Clump-size defines the area of a clump. In one embodiment, the clumpsize is defined in world space, a space that a user typically referenceswith respect to a size of an object. In one embodiment, clump densitytakes priority over clump size, such that if there are many clumps andmost of them overlap, the clump size cannot be maintained, since a clumphair can be a member of only one clump. If both clump density and sizeare small, many hairs between the clumps will not be clumped.

Referring to FIG. 8, to determine clump membership of each final hair(i.e., what clump each hair belongs to, if any), the clump of thespecified clump-size is converted into u-radius and v-radius componentsin parametric surface space at each clump-center hair location, step800. Each hair is evaluated at steps 805, 810 to determine whether itfalls within the u, v radius components of a corresponding clump-centerhair. If the hair is not within the u, v radius components, the hair isnot a clump hair, step 815 and the process continues, step 830, with thenext hair. If the hair is within the u, v radius components, at step 820the clump-center hair's index is referenced with the hair. In addition,a clump rate and clump percent is assigned, step 825.

A number of variations are contemplated. A clump-size noise parametermay be introduced to produce random variations in the size of theclumps. Feature (texture) maps for a clump-size can be created andspecified by the user, one per surface patch, to provide local controlof the radii used at steps 805, 810. In this embodiment, the globalclump-size input parameter is multiplied for a particular clump (clumpcenter hair) at (u,v) on a surface patch with the correspondingnormalized (s,t) value in the clump-size feature map for that surface.Also, a static clump-area feature map can be provided to limit clumpingto specified areas on surface patches rather than the whole model.

In one embodiment a clump-percent and clump-rate value is assigned toeach clump hair (step 825). In one embodiment, the values for both rangebetween [0,1], and are used subsequently to reorient clump hairs, step835, as described below.

Clump-percent specifies the degree of clumping for a clump hair. Forexample, a value of zero indicates that the hair is not clumped at all,i.e., it is like a “dry” hair. A value of one indicates that the hair isfully attracted to its clump-center hair, i.e., the tip of the hair (itsdistant control vertex) is in the same location as the tip of theclump-center hair.

Clump-rate defines how tightly a clump hair clumps with itscorresponding clump-center hair. For example, a value of zero indicatesthat the clump hair is linearly increasingly attracted to itsclump-center hair, from the root to the tip. A clump-rate value closerto one indicates that the hair's control vertices closer to the root areproportionally more attracted to corresponding clump-center hairvertices than those closer to the tip, which results in tighter clumps.Examples of different values for clump-percent and clump-rate are givenin FIG. 9.

At step 835, the control vertices (except the root vertex) of each clumphair are reoriented towards corresponding clump-center hair verticesfrom the clump hair's dry, combed position determined at steps 200, 210,215, 220, and 223.

In one embodiment, this process is performed at each frame. In oneembodiment, the default value for number of control vertices (CVs) is 3(4 minus the root vertex), and the index for the current control vertexi ranges from 1-3. In one embodiment, the reorientation is determined asfollows:clumpHairCV[i]=clumpHairCV[i]+delta*(clumpCenterHairCV[i]-clumpHairCV[i])delta=clumpPercent*(fract+clumpRate*(1−fract)); wherefract=i/numberOfCVs; clumpHairCV[i] represents a clump hair vertex;clumpCenterHairCV[i] represents a corresponding clump center hairvertex; i represents an index to a current control vertex; numberofCVsrepresents the number of control vertices of a clump hair; clumpPercentrepresents clump-percent; and clumpRate represents the clump-rate.

Both clump-percent and clump-rate parameters can be locally controlledvia feature maps similar to the feature maps described above withrespect to clump-size. Both values can also be animated or changed overtime to provide continuous control for dry-to-wet-to-dry fur looks. Thisis illustrated by FIGS. 10 a, 10 b, 10 c and 10 d which illustrate fourframes from an animated clump-percent and clump-rate sequence. In theimage of FIG. 10 a the clump-percent and clump-rate are both zero andmay represent dry, combed hair. In the image of FIG. 10 b, clump-percentis 0.7 and clump-rate is 0, which results in a slightly wet look. In theimage of FIG. 10 c, clump-percent is 1.0 and clump-rate is 0.3, whichresults in a wet look. In the image of FIG. 10 d, clump-percent andclump-rate are both 1.0, which produces a very wet look.

Animated area clumping is desirable to simulate spouts of water orraindrops hitting the fur and making it increasingly wet. At step 230,FIG. 2, animated clumping is performed. In one embodiment, the animatedclumping areas are defined in an animation system.

One embodiment of the process is described with reference to FIG. 11. Inone embodiment, clumping areas are defined by particles hitting surfacepatches. Other embodiments may use alternate techniques for generatinganimated clumping areas. At step 1100 a global static area clumpingprocess is performed on all hairs. This step identifies clumping regionsand corresponding clump center hairs and clump hairs. As explained belowthis information is used in the animated clumping process. In oneembodiment, the global static area clumping used is that described abovefor static area clumping.

At step 1102, one or more emitters that generate the particles aredefined. The use of emitters to generate particles is known in the artand will not be discussed in detail herein. In one embodiment, theemitters define the rate generated and spread of the particles across asurface.

At step 1105, at each frame for each particle generated that hits thesurface, the surface patch the particle hits is identified, step 1110.In one embodiment, particles generated in prior frames are carriedthrough subsequent frames such that the particles are cumulative.

For each particle that hits a surface patch, including those particlesgenerated in prior frames, a circular animated clumping area is created,step 1115, on the patch at that (u,v) location, with clump-percent,clump-rate, and animated clumping area radius determined by a creationexpression executed at the frame where the particle hits the surface sothat when a particle hits the surface at that time (i.e., at the frame),the clump-percent may be set to zero and the radius may be defined to aspecified value perhaps adjusted by a random noise value. Thus, theexpression may be defined to provide the desired “wetness” effect.

The radius of the circular clumping area defined is converted into acorresponding u-radius and v-radius similar to the clump size discussedabove. Runtime expressions executed at each frame define clump-percentand clump-rate, thus determining how quickly and how much the fur “gets”wet. For example, one runtime expression may be: MIN(FrameNumber*0.1, 1)such that as the frame number increases, the hair appears increasinglywet.

Each clump center hair of a clump (determined at step 1100) is thenevaluated to determine if it falls within the animated clumping area,step 1120. To determine whether a clump falls within an animatedclumping area, at each frame it is checked as to whether the (u,v)distance between the clump-center hair of the clump and the center ofthe animated clumping area is within the (u,v) radius parameters of theanimated clumping area. For clumps that are located in overlappinganimated clumping areas, the values for clump-percent and clump-rate areadded resulting in the generation of wetter fur.

If the clump center hair is within the animated clumping area, step1125, the corresponding clump is flagged with an animated clumping flagsuch that the clump hairs are subsequently reoriented to reflect theanimated clumping effect. Alternately, each clump hair of the clump mayhave an animated clumping flag which is set if the corresponding clumpcenter hair is determined to be within an animated clumping area. Inaddition, an animated clump-rate value and an animated clump-percentvalue are assigned to the clump hairs that are identified to be withinan animated clumping area in accordance with a runtime expression. Inone embodiment, the values for clump-percent and clump-rate for eachclump within an animated clumping area are replaced with thecorresponding values for the animated clumping area at each frame. Asanimated clumping areas may be much bigger than a clump, an animatedclumping area may contain several individual clumps. Each clump isevaluated, step 1140, for each particle, step 1145.

It should be noted that animated clumping areas could straddle surfacepatch boundaries. For example, the center of an animated clumping areamay be located on one surface patch, but the area may be located on oneor more other patches. Since the animated clumping areas are typicallydefined and therefore associated with the surface which contains thecenter of the animated clumping area, i.e., the position where theparticle hit, portions of a clumping area straddling neighboring patchesmay be overlooked. This could lead to discontinuities in clumping of thefinal fur.

In one embodiment, this potential problem is addressed. Whenever a newparticle hits a surface and the (u,v) radii exceed the boundaries ofthat surface; an additional (u,v) center and (u,v) radii is generatedfor the animated clumping areas affecting neighboring patches. Thus, forexample, if the clumping area covers portions of two neighboringpatches, a corresponding (u,v) center and radii are generated for eachneighboring patch to provide additional animated clumping areas forevaluation at steps 1120-1140

At step 1150, for each frame, the clump hairs of clumps that are withinthe animated clumping areas are reoriented. Thus, clump hairs areselectively adjusted if they are within an animated clumping area. Inone embodiment, clumping is restricted to the animated clumping areas ateach frame, so that final hairs of clumps outside the animated clumpingareas are normally produced as “dry” hairs.

At step 1155, if more frames are to be processed, the process continuesagain at step 1105. Thus the animated clumping process is performedacross multiple frames to provide animated effects.

Referring back to FIG. 2, step 235 may be applied to generate the effectof hair breaking or dry fur clumping by breaking up the groomed fur coatalong certain lines (fur tracks or break line) on the underlying skin(surfaces). As described below, this process may include two kinds ofhair breaking: symmetric and one-sided. In symmetric breaking, hairs onboth sides of a fur-track “break” towards that track, whereas inone-sided breaking, hairs on one side of the track break away from thetrack.

In one embodiment, fur tracks are specified as curves on surfaces in ananimation system. Each track has a radius, break-percent and break-ratefor symmetric and one-sided breaking, and an additional break-vector forone sided breaking. The final information generated is output intobreaking files that are subsequently accessed to reorient the affectedhairs.

One embodiment of the hair breaking technique is illustrated by FIG. 12a. At step 1200 the fur tracks are defined. The fur tracks may bedefined similar to clumps by defining a (u,v) break radii. At step 1205the break line hairs (hairs which lie on or are very close to thefur-track curve defined by the curve defined for the fur track) arecomputed. Using the break line hairs and break radii, at steps 1215,1220, each hair is evaluated to determine whether the hair lies withinthe (u,v) break radii on both sides of the break line hairs in case ofsymmetric breaking, or to one side specified by the break vector (thebreak vector side) in case of one-sided breaking. For each hair withinthe space specified by the radii, referred to herein as a break hair,the corresponding break line hair (hair on the fur track) is thendetermined as the one closest to it. The hairs are labeled as break linehairs, break hairs with indices to their corresponding break line hairs,or normal hairs that do not reside within the areas specified by thebreak.

It should be noted that for instances of one-sided breaking, each breakhair is now reoriented “away” from its corresponding break line hair inthe direction of the break-vector, rather than “towards” the break linehair. Examples of symmetric and one-sided breaking are shown in FIG. 12b.

The break hairs are reoriented with respect to their corresponding breakline hairs, step 237. For symmetric breaking, this process is analogousto the process performed for clump hairs discussed earlier. However, forbreak hairs, the break-percent and break-rate values are used in placeof the clump-percent and clump-rate used for clump hairs. For one-sidedbreaking, break hairs are repelled, as opposed to attracted to thebreak-line hairs according to the break-percent and break-rateparameters.

The breaking effect is illustrated by FIGS. 12 c, 12 d 12 e and 12 f.FIG. 12 c illustrates an object 1250 having break line hairs 1252, 1254.FIG. 12 d shows the resultant effect on the object for symmetricbreaking. FIGS. 12 e and 12 f illustrate one-sided breaking along breakline hairs 1256-1270.

At step 245, FIG. 2, a decision is made as to whether multiple passes ofthe process are to be performed. The coat of most furred animals iscomposed of a fuzzier, thinner shorter layer of hairs called undercoat,plus an overcoat of longer and thicker hairs. Step 245 illustrates thecapability to perform a two-(or multiple)-pass process, whereby steps210, 215, 220 and 223 (and optionally 225, 230, and 235) are executedmore than once, producing a different set of hairs at each pass. Thesesets or layers are then processed and combined at render-time (step250). The effects can be seen by reference to FIGS. 13 a, 13 b and 13 c.FIG. 13 a is a representation of an undercoat generated in accordancewith the teaching of the present invention. FIG. 13 b represents arepresentation of the overcoat and FIG. 13 c represents the combinedimage consisting of the undercoat and overcoat.

As illustrated by step 250, the clumped hairs represented by theircontrol vertices are rendered into a series of two-dimensional images tocreate lifelike dry and wet hair looks. In one embodiment, the processfunctions to project a three-dimensional hair geometry onto atwo-dimensional image plane from the perspective of a particular pointof view.

In order to render large amounts of hair quickly and efficiently, thegeometric model of each hair may be kept simple. As explained above, ahair is represented by a parametric curve having a determined number ofcontrol vertices (in one embodiment, the default is four).

In one embodiment, the process employs known rendering technology toproduce the hairs described by the corresponding control vertices. In analternate embodiment customized modules are added to realistically“shade” the hairs. This may be accomplished by assigning an intensity ofcolor at each point along or on a hair, wherein points along a hair maybe defined as the pixels which compose the hair.

During the rendering of the hairs, a width is added for each hair totransform it into a narrow ribbon that is always oriented towards thecamera or view point. The shading process properly shades these ribbonprimitives to more realistically present them as thin hairs.

One embodiment of the shading process is set forth in the flow chart ofFIG. 14. At step 1400, each hair is processed. At step 1405, for eachhair, the surface normal at the base of the hair is mixed with thenormal vector at the current point on the hair in order to obtain ashading normal at the current point on the hair. In one embodiment, thehairs are rendered as a series of points or pixels on a display. Thus,the current point is one of the pixels representing a hair.

The shading process may be applied at multiple points along the hair. Inone embodiment, the amount with which each of these vectors contributesto the mix is based on the angle between the tangent vector at thecurrent point on the hair, and the surface normal vector at the base ofthe hair. The smaller this angle, the more the surface normalcontributes to the shading normal.

At step 1410 the intensity of the hair at the current point on the hairis determined using the shading normal at that point. In one embodiment,a Lambertian model is used to calculate these intensities. Using thisapproach provides the benefit of allowing the user to light theunderlying skin surface and receive predictable results when fur isadded. This approach also accounts for shading differences betweenindividual hairs, and differences in shading along the length of eachhair.

In order to obtain realistic shadows on the fur coat, shadow maps areused. The use of shadow maps is known in the art and will not bediscussed further herein. However, incorporating the hair into theshadow maps may generate several unwanted side effects. One problem isthat of dark streaking on brightly lit fur because of the furself-shadowing. Dark streaks look wrong on brightly lit fur becausenormally light bounces off the skin and hair to prevent dark shadows onbrightly lit fur.

In order to minimize the dark streaking effects, in one embodiment, thehairs for the shadow maps are shortened based on certain criteria, step1415. For example, the length and density of the hair may dictate thepercentage to shorten the hair. By selectively shortening hairs for theshadow maps, the hair self-shadowing effect is minimized while stillproducing a broken up shadow on the terminator lines for lights fallingon the fur.

Back lighting is achieved in a similar fashion using a shadow map foreach light located behind the furred object, and again shortening thehair on the basis of density and length in the shadow map renderprocess. In one embodiment, a lighting model for hairs also allows eachlight to control its diffuse fall-off angles. Thus, lights directlybehind the furred object can wrap around the object. Using theselighting controls together with shadow maps reasonable back lightingeffects are achieved.

In one embodiment, the shading for clumped hairs is modified. In oneembodiment, two aspects of the hair shading may be modified. First, theamount of specular on the fur is increased. Second, clumping isaccounted for in the shading model. Geometrically, as explained earlier,fur is modeled in clumps to simulate what actually happens when fur getswet. In the shading model, for each hair and for each light, the side ofthe clump the hair is on with respect to the light's position isdetermined, and the hair is either darkened or brightened based on theside the hair is on. Thus, hairs on the side of a clump facing the lightare brighter than hairs on a clump facing away from the light.

Additional Embodiments

Further embodiments of the invention relate to additional features addedto the animal fur and human hair pipeline of FIG. 1 b. In particular,these additional features to the pipeline are directed to producing awide variety of stylized and photorealistic fur and hair looks fordigital characters.

Turning now to FIG. 15, FIG. 15 is a block diagram illustrating ahair/fur pipeline 1500, similar to the previously-described pipeline ofFIG. 1 b, but that includes additional and differing functionality.

As before, input, is received by surface definition module 1550. Surfacedefinition module 1550, as previously described, defines surfaces andcontrol hairs of the object to be rendered. Further, as previouslydescribed, control hair adjustment module 1555 adjusts control hairs toprovide such functionality as combing and seamless hairs across surfaceboundaries.

In addition to the previously-described pipeline of FIG. 1 b, a hairmotion compositor module 1557 has been added to hair/fur pipeline 1500to provide for editing and combining different hair animations, as willbe described in detail hereinafter. Interpolation module 1560, aspreviously described, may be utilized to interpolate hairs acrosssurfaces using control hairs.

Additionally, an effects module 1565 may be utilized as part of thehair/fur pipeline 1500 to provide various effects to hair and fur suchas clumping, breaking, wave effects, weave effects, etc., as will bedescribed in more detail hereinafter. Further, an optimization module1567 may be utilized with hair/fur pipeline 1500 to providemethodologies to improve render times, as will be described in moredetail hereinafter.

Shading, backlighting, and shadowing module 1570 may be utilized inhair/fur pipeline 1500 to provide shading, backlighting, and shadowingeffects to hair/fur and display module 1575 may be utilized to renderand display the final output of the object with the hair/fur surface, aspreviously described.

Moreover, as previously described with reference to FIG. 1 a, it shouldbe appreciated that the hair/fur pipeline 1500 may be implemented with acomputer system having a central processing unit (CPU), memory,input/output (I/O), etc., which may be coupled to a storage device suchas a disk drive or other device. Further, the computer system mayinclude a keyboard or other user input devices as well as a display thatmay be used to display user interfaces and the rendering of hair/fur inaccordance with the embodiments of the present invention.

In particular, embodiments of the invention are directed to techniquesimplemented by the previously-described pipeline 1500 of FIG. 15 relatedto such items as: special combing tools, different final hairinterpolation algorithms, and a general application program interface(API) to perform customizable per-hair calculations in addition torendering each final hair. Further techniques relate to: a method andsystem for editing and combining different hair animations referred toherein as a Hair Motion Compositor (HMC) and hair optimizationstrategies to improve render times. In one embodiment, the HMC may beimplemented by Hair Motion Compositor module 1557 of hair/fur pipeline1500 and the optimization strategies may be implemented by optimizationmodule 1567 of hair/fur pipeline 1500.

Digital animals, humans, and imaginary creatures are increasingly beingincorporated into movies, both live-action and computer-animated. Inorder to make them believable, many of these characters need persuasivehair or fur. In a production environment, where quality is paramount, apipeline to generate hair must not only work, but also needs to bepractical, robust, flexible, efficient, and powerful.

Described herein are tools and techniques which facilitate the creationof specific hair and fur looks to satisfy the appearance and expressionthat a director might choose for a particular show and its characters.Solutions are described which make it possible to generate convincinganimal fur, produce believable human hair, and closely match the hair ofreal actors.

Described herein are improvements to the hair/fur rendering pipeline ofFIG. 1 b, which address and improve three of the most complex andtime-consuming areas in the digital hair creation process: lookdevelopment (combing), hair animation and shots, and rendering largenumbers of hairs.

It should be noted that when modeling hair geometrically, problems arisewith respect to human hair that are slightly different from those withanimal fur. Specially, longer human hair requires much moresophisticated combing and animation tools. For animal fur, the renderingstage needs to be optimized as there are millions of individual hairstrands compared to around 100,000 to 150,000 for humans.

As will be described herein, the techniques employed by hair/furpipeline 1500 relate to producing a wide variety of hairstyles, fromshort animal fur to long human hair. In particular, hair/fur pipeline1500 includes features such as: tools for modeling, animating, andrendering hair; allowing a small, arbitrary number of control hairs todefine basic comb and animation features; and techniques forinterpolating final rendered hair strands from these control hairs.

It should be noted that conventional 3D animation software, such as MAYAavailable by AUTODESK, may be utilized to provide conventional viewingfunctionality. Additionally, the RENDERMAN software package available byPIXAR may be utilized to aid in rendering final hair. These softwarepackages are well known to those in the computer graphics arts.

Additional combing and control hair editing tools implemented by thehair/fur pipeline are described hereinafter. For example, in oneembodiment, a basic guide-chain attached to a control hair may be usedin either a forward or inverse kinematic mode to define a control hair'sshape while approximately maintaining its length. It can also apply thesame deformation to other selected control hairs-either in world orlocal space. Further, an intuitive cutting tool may be utilized in whicha user can sketch a curve in an orthographic view, which is then used tocalculate intersections with selected control hairs and cuts them atthose points.

In one particular embodiment, a combing tool is disclosed that allowsfor selected control hairs to be clumped together in a controlledmanner.

With reference to FIGS. 16 a and 16 b, FIGS. 16 a and 16 b are diagramsthat illustrate the clumping of control hairs with possible variationsalong the control hairs. As shown in FIG. 16 a, a first set of controlhairs 1602 and a second set of control hairs 1604 are illustrated asextending normally from the head of a creature 1606. A clump profilewindow 1610 includes a user-definable clump profile curve 1611implemented with control hair adjustment module 1555 of hair/furpipeline 1500 that may be applied to the first set of control hairs 1602so that the control hairs are deformed in such a fashion, as shown bythe deformed control hairs 1615, such that they approximateuser-definable clump profile 1611 as can be seen in FIG. 16 b.

Fill-Volume Techniques

In one embodiment, surface definition module 1550 of the hair/furpipeline 1500 may be used to generate a shape defining a surface and anassociated volume. Control hair module 1555 may be used to fill thevolume with control hairs and interpolation module 1560 may be used tointerpolate final hair strands from the control hairs. These techniqueswill be described in more detail below.

In particular, a fill-volume tool may be used to quickly fill anenclosed surface with randomly placed control hairs. This may be usefulin generating hair to fill “hair volumes” such as volumes defining aponytail often provided by modelers to describe the rough hair look of acharacter.

Turning to FIG. 17, FIG. 17 is a flow diagram illustrating a process1700 to implement a fill-volume function. As shown in FIG. 17, at block1710, a shape is generated that defines a surface and an associatedvolume. For example, this may be accomplished utilizing surfacedefinition module 1550 of hair/fur pipeline 1500. Next, the volume isfilled with randomly placed control hairs (block 1720). For example,this may be accomplished utilizing control hair adjustment module 1555of hair/fur pipeline 1500. Lastly, final hair strands are interpolatedfrom the control hair strands (block 1730). For example, this may beaccomplished utilizing interpolation module 1560 of hair/fur pipeline1500.

Further, an example of this may be seen in FIGS. 18 a-c, where braidsare simply represented as surfaces by a modeler, and then filled withhair. In particular, FIG. 18 a-c show the generation of braid shapesdefining surfaces and associated volumes (FIG. 18 a), the filling ofthese volumes with randomly placed control hairs (FIG. 18 b), and theinterpolation of final hair strands from the control hairs (FIG. 18 c).

As shown in FIG. 18 a, at first, three braided cylinders 1802, 1804, and1806 are generated each of which defines a shape and an associatedvolume. Next, as shown in FIG. 18 b, the fill-volume function isutilized to generate control hairs 1810, 1812, and 1816, respectively.Lastly, as can be seen in FIG. 18 c, the final braids of hair 1820,1822, and 1826 are interpolated from the control hairs and are rendered.

Additional control hairs may be automatically interpolated from existingones and final hair strands may also be automatically interpolated fromexisting ones utilizing interpolation algorithms similar to thosedescribed below. Additionally, the shape of newly inserted control hairsmay be blended between selected existing control hairs.

In one embodiment, the hair/fur pipeline 1500 may utilize differentalgorithms to interpolate final hair strands from control hairs as wellas other control hairs from existing control hairs (e.g. utilizinginterpolation module 1560) based on different coordinate schemes.

An example of this interpolation methodology may be seen with referenceto FIGS. 19 a-c. As can be seen in FIGS. 19 a-c, side views of adeformed surface 1900 are shown with identical control hairs 1902. Byutilizing this interpolation methodology, the number of control hairsmay be kept small across different applications.

For example, FIG. 19 a shows a first interpolation method for theinterpolation of final hair strands 1904 from control hairs 1902 in a“world space” coordinate frame, in which the final hair strands 1904 donot automatically follow surface deformations. This is useful for plantsor long hairstyles that should not follow surface (skin) deformations.

FIG. 19 b shows a second interpolation method for the interpolation offinal hair strands 1904 from control hairs 1902 in a “local space”coordinate frame. In the “local space” scheme, the final hair strands1904 follow surface deformations automatically. This is more natural forshorter hair or fur.

Lastly, FIG. 19 c shows a third interpolation method for theinterpolation of final hair strands 1904 from control hairs 1902 inwhich the final hair strands 1904 remain within the convex hull ofinfluencing control hairs 1902. This scheme may be useful, for instance,for long, un-clumped human hair dropping from the top and sides of thecurved scalp. It should be noted that in the local and world spacemodes, interpolated final hair strands 1904 may appear longer thancontrol hairs 1902 whereas in the convex hull mode they do not.

Further, hierarchical clumping capabilities of final hair strands may beimplemented utilizing the hair/fur pipeline 1500 (e.g. utilizing effectsmodule 1565). Final hairs may belong to manual clumps directly placed bya user or to procedurally generated auto and minor clumps. Auto-clumphairs may occur either inside or outside of manual clumps whereas minorclump hairs may only reside in manual or auto clumps. These techniquesmay be applied for wet hair looks and for customizing the look of tuftsof dry hair.

A further characteristic of the hair/fur pipeline 1500 is that amultitude of parameters or effects such as wave, weave, (rotationsaround the root of the hair), shorten-tip and wind may be implemented,for example, by effects module 1565 of hair/fur pipeline 1500. Theseeffects may be applied directly to final hairs after the interpolationbetween control hairs.

Examples of these wave, weave, and wind effects are shown in FIGS. 20a-c. In each of the cases of FIGS. 20 a-c, control hairs 2002 are shownas pointing straight up. For example, FIG. 20 a illustrates no effect onthe final hairs 2004. FIG. 20 b indicates some wave and weave effectbeing applied to the final hairs 2004. Lastly, FIG. 20 c illustrates awind effect being applied to the final hairs 2004.

Also, effects may be applied selectively to different hair “types” suchas manual clump member hairs or un-clumped hairs, before or afterclumping. In addition, independent hair parameters or effects can beused in combination with multiple final hair layers with possiblydifferent sets of control hairs. For animal fur, undercoats andovercoats may be generated this way and complex human hair may be brokenup into several distinct layers such as base, stray, and fuzz.

Geometric Instancing

Another embodiment of the invention relates to arbitrary geometryinstancing. Arbitrary geometry instancing provides a feature thatleverages off the hair/fur pipeline's 1500 ability to instantiate,animate, and control the style or look of final rendered hair primitive,as previously described. In this regard, a general application programinterface (API) may be provided in conjunction with the hair renderingfunctions such that an application developer can obtain per hairfollicle information from the hair system, and replace the rendering ofhair with other operations.

For example, in one embodiment, hair pipeline 1500 may be used togenerate a user-selected geometry based upon a hair location for atleast one hair with respect to a surface. Surface definition module 1550may be used to define the surface. Display module 1575 may be used to:process a user-selected geometry and render the user-selected geometryat the hair location on the surface in place of the at least one hair.

In particular, instead of outputting hair follicles of the finalrendered primitive, a hook into the hair/fur pipeline 1500, e.g., atdisplay and rendering module 1575, is provided to allow the user tooverride the rendered geometry and instead of rendering a hair,rendering another geometric shape. Thus, instead of populating, forexample, a human head with hair, a human head with thorns may begenerated. Or, instead of a field full of hairs, a field full offlowers, grass, or trees may be populated.

Further, it should be noted that instantiating the exact same object(e.g., flowers or trees) does not work well when attempting to design aspecific landscape because all the flowers or trees would be identical.Therefore, different kinds of primitives may be modeled and instantiatedat render time to replace each final hair.

Instead of completely discarding the final shape of the hair, the hairfollicle can be used to represent the axis of deformation around whichthe instanced geometry may be deformed according to the shape of thehair. This way, all of the advantages of all the various effects (e.g.,clumping, breaking, wave, weave, wind, etc.) previously described as torendering hair may be utilized on rendering the other selected geometry(e.g., flowers, trees, etc.).

Turning now to FIG. 21, FIG. 21 is a flow diagram illustrating a process2100 to implement geometric instancing, according to one embodiment ofthe invention. At block 2105 it is determined whether or not thehair/fur pipeline 1500 has entered a render stage. If not, the processis ended (block 2110). However, if the render stage has been entered, atblock 2115, it is determined whether or not a user has decided tooverride the hair rendering operation to instead utilize the geometricinstancing function. If not, the process is ended at block 2120.

On the other hand, if the user has decided to override the hairrendering process and to utilize the geometric instancing functioninstead, then at block 2125 the hair follicle information in obtained.Next, at block 2130, based upon the user selected geometry, the hairfollicles upon the surface are instead populated with the user-selectedgeometry 2130 (i.e., instead of hair/fur). Then, at block 2135, the hairshape previously determined for the hairs are applied to deform theuser-selected geometry.

An example of this can be seen in FIGS. 22 a and 22 b. As shown in FIG.22 a, a plurality of hairs 2210 are shown as being rendered on a surface2212. Each of the hairs 2210 has hair follicle information (e.g., itsrelative position on the surface 2212) and associated therewith an axialdeformation framework (e.g., defining the shape of the hair) and relatedcontrol information as to shape features such as curve, bend, rotation,twisting, etc. (as previously-described).

However, with embodiments of the invention, as shown in FIG. 22 b, auser may decide to render another geometric object (e.g., a flower)instead of a hair. For example, a flower may be randomly selected from aset of three modeled flowers and may be deformed by the shape of thecorresponding final hair.

As shown in FIG. 22 b, a plurality of flowers 2220, 2222, and 2224, areshown as replacing the hairs utilizing the same hair follicleinformation and the same shape information. It should be appreciatedthat flowers are only given as an example and that virtually anygeometric shape may be used in lieu of a hair shape. Thus, newgeometries may be instanced and deformed utilizing the hair/fur pipeline1500 thereby providing a powerful and a unique way of rendering any typeof geometric shape.

Hair Motion Compositor System

When dealing with hair animations and dynamic simulations in aproduction context, the need to combine different motion results arisesquite often. If part of the hair is perfect in one simulation, but therest of the hair looks better in another, it is easier to pick andchoose which parts of each simulation we want to keep than it is tofigure out the right settings to get all the hair to move in a desiredfashion.

As will be described hereinafter, in one embodiment, a hair motioncompositor provides a system and method that allows a user to combineand modify various control hair animations by building a network ofnodes and operations. For example, in one embodiment, hair motioncompositor module 1557 of hair/fur pipeline 1500 may be utilized toimplement the hair motion compositor.

For example, in one embodiment, hair pipeline 1500 utilizes surfacedefinition. module 1550 to define a surface and a control hair and hairmotion compositor module 1557 combines different control hair curveshapes associated with the control hair and the surface. In particular,the hair motion compositor module 1557 generates a static node defininga static control hair curve shape; generates an animation node definingan animation control hair curve shape; and combines the static controlhair curve shape of the static node with the animation control haircurve hair shape of the animation node to produce a resultant controlhair curve shape for the control hair.

Nodegraph Basics

In one embodiment, the hair motion compositor (HMC) is a directedacyclic graph of nodes, wherein nodes can represent either animations oroperations applied to animation. The connection between the nodesrepresents the data flow of animated control hair curve shapes.

Simple Nodes

Creating an HMC setup for a character or object typically assumes thatthere is an existing static comb of control hairs that is used as abasis. A HMC setup typically requires two nodes: a static node(containing the initial, non-animated shape of the control hair curves)and a control node.(representing the final result—which is the output ofthe node graph). Additional nodes representing control hair curve shapesmay be inserted into the graph. These additional nodes may be termedanimation nodes.

Turning to FIG. 23, FIG. 23 is a diagram illustrating a simple graph ofa static node 2302 connected to an animation node 2304 and a finishedcontrol node 2306. As can be seen in FIG. 23, the input to the animationnode 2304 is the static combed shape from static node 2302. The user canapply tweaks and key frames through an animation set of control haircurves provided by animation control node 2304 to offset the staticshape to produce the final combed hair result of control node 2306.

Blend Nodes

Blend nodes may be utilized for compositing features. A blend node maybe defined as an operation node that takes two inputs and combines themtogether to form a single output. For example, control hair curves fromeach input may be blended together rotationally (to maintain curvelength) or positionally (for linear blending).

Additionally, a blend factor parameter for the blend node may be used tocontrol how much each of the inputs should be used. For example, a valueof 0 designates the total use of input node A and a value of 1designates the total use of input node B; wherein the blend nodeprovides smooth interpolation of the control hair curve shapes of inputnodes A and B for all of the values in between.

Turning now to FIG. 24, FIG. 24 is a diagram illustrating a process ofutilizing a blend node 2403. Concurrent reference may also be made FIGS.25 a and 25 b, which illustrate the difference between rotational andpositional blending, respectively. In particular, FIG. 24 shows a blendnode 2403 blending inputs from static node 2402 and animation node 2404to obtain a resultant output shape of control node 2406. In thisexample, blend node has a blend factor of 0.5.

More particularly, as can be seen in FIG. 25 a, when rotational blendingis selected by blend node 2403 with a blend factor of 0.5, resultantcontrol hairs 2510 are obtained based upon input static control hairs2520 and animation control hairs 2530. The advantage of rotationalblending is that it preserves the lengths of the resulting controlhairs.

As can be seen in FIG. 25 b, when positional blending is selected byblend node 2403 with a blend factor of 0.5, resultant control hairs 2555are obtained based upon input static control hairs 2560 and animationcontrol hairs 2570. The advantage of positional blending is that theresulting control hair shapes are more predictable than with rotationalblending.

By default, blend node 2403 may apply the same blend value to allcontrol vertices (CVs) of every input control hair curve. For per-CVcontrol, a user may build a function curve specifying what blend factorvalue to use at every CV. For example, this may be used to cause thebase CV motion to come from a first input, and the tip CV motion to comefrom a second input. Typically the same blend factor is applied to everycontrol hair curve of the inputs, even when using per-CV function curve.

To specify per-hair blend values, a user may utilize a blend ball thatidentifies regions in three-dimensional space. A blend ball may be madeof two concentric spheres with an inner and outer blend factor value,respectively. If the inner value is 0 and the outer value is 1, allcontrol hair curves within the inner sphere will get their animationfrom the first input, and all control hair curve outside the outersphere will get their values from the second input, with a smoothinterpolation in between.

An example of this type of blend ball 2600 is illustrated in FIG. 26. Inessence, blend ball 2600 is utilized to localize the effect of the blendnode. As can be seen in FIG. 26, blend ball 2600 includes an innersphere 2602 and an outer sphere 2604. Further, FIG. 26 shows the effectof blend ball 2600, via inner sphere 2602 and outer sphere 2606, uponblending animation control hairs 2607 with input static control hairs2608, and further shows the resultant output control hairs 2610.

Dynamic Solver

It should be appreciated that other than specifying typical simulationsettings (stiffness, damping, etc.), that a lot of initial effort goesinto specifying a good goal shape that is to be fed as an input to asolver (such as key poses the user may want the control hair curves to“hit”). However, eventually most of the time is spent dealing with theresults of one or more simulations.

Since the dynamic solving process itself is often time consuming andoutside of direct user control, it is important to minimize the numberof simulations that need to be run to achieve the desired result.

For example, in one embodiment, the MAYA hair dynamic solver may be usedfor hair simulations. However, it should be appreciated that the HMCsystem previously described is dynamic solver-agnostic. In essence, thesolver appears as a single node in the node graph.

With reference to FIG. 27, FIG. 27 illustrates a dynamic setup includinga dynamic solver node (represented by solver node 2706 and dynamic node2708). The initial static comb node 2702 is connected to the solver 2706through animation node 2704 to be used as a goal, but the final resultis blended (by blend node 2712) between the output of the dynamicsimulation node 2708 and the goal. Control node 2710 is the resultantoutput.

By default, the blend is set to 0 to be 100% dynamic. However, toquickly make the hair stiffer (a common artistic request), the blend canbe gradually increased to blend back to the static goal without runningnew simulations.

Depending on the settings used and the complexity of the scene, dynamicsimulations can take a long time, so that they often may be run on aRENDERFARM in order to free up a user's computer for other work. Inorder to accommodate this, a cache-out node 2720 connected to controlnode 2710 may be utilized to write to a storage device whenever a nodeis connected to it. The cache-out file can then subsequently be readback in with a cache-in node, as will be described. It should beappreciated that a cache-out node may be applied to any node of thesystem. Further, it should be appreciated that the storage device may bea hard disk drive or any other sort of storage device.

Volume Nodes

Very often, especially for computer graphic feature productions, proxysurfaces representing hair are modeled and key framed at certainkey-poses by the animation department in order to represent how the hairshould move in a given shot.

For example, a ponytail of a character may be approximated by abulged-out tube volume. In order to accommodate this functionality, avolume node may be utilized that in effect binds control hair curves sothat they follow the animation of one or more volumes. Another use ofvolume nodes may be to offset the result of a dynamic simulation toproduce a desired result. In this case, it may be impractical to use ahand-animated volume since it does not automatically follow the dynamiccontrol hairs. Instead, a volume node may be used that provides its ownvolume which may be built-in on-the-fly as the convex hull of thecontrol hair curves to be deformed. A user may then modify the volume tooffset the animation. This is much easier than modifying the controlvertices of individual hairs.

FIG. 28 a is a diagram that demonstrates an automatically-generatedcylindrical volume node 2804 that surrounds control hair curves 2802.FIG. 28 b is a diagram showing control hair curves 2810 that have beenoffset after the user has edited the control hair curves with the volumenode 2804. As can be seen in FIGS. 28 a and 28 b, the control hairs 2802may be easily modified by the volume node 2804 as shown by modifiedcontrol hair curves 2810.

Super Hair Nodes

Super hair nodes provide a mechanism similar to the volume nodes byshaping or offsetting hair animation through a simplified proxygeometry. However, the proxy geometry in this case is a curve attachedto the surface like all of the other hairs. A super hair node may beutilized to control a plurality of control hairs both at the combingstage to shape static hairs and in a shot to alter animated hairs.

In one embodiment, a super hair node may have two modes of operation:absolute and relative. In absolute mode, a single controller curve isused by the super hair node to dictate the exact shape and animation ofthe control hairs. A user may choose to only partially apply the effectwith a weight parameter whose value varies between 0 and 1. The superhair node also has the option to match the shape of the controller curvein world-space or to have the deformations applied in the local space ofeach control hair curve.

Turning to FIG. 29, FIG. 29 is a flow diagram illustrating a process2900 of super hair node processing. At block 2910 a user selects a superhair node. At block 2915, a user selects whether or not to utilizeeither an absolute or a relative mode. If absolute mode is selected, atblock 2920, a weight parameter may be selected and at block 2925 worldor local space may be selected. At block 2930 the controller curve isapplied to the control hairs.

On the other hand, if during process 2900, relative mode is selected, auser likewise selects a weight parameter (block 2940) and selects worldor local space (blocks 2945). In the relative mode, however, both thecontroller curve and the base curve are applied (block 2950) to thecontrol hairs.

In the relative mode, both a controller curve and a base curve are used,and only the difference between the two is applied to the control haircurves. This method is typically used when modifying control hairs thatalready have some incoming animation. It should be noted that the superhair node has no effect if the controller and base curve match exactly.When creating a super hair node, the controller and base curves arecreated straight up by default. For offsetting animated hairs, bothcurves can optionally take on the average shape of the control hairs.Tweaking the controller curve from there is then much more intuitive.

Turning briefly to FIGS. 30 a and 30 b, super hair operations inlocal-space and world space, respectively, are illustrated. As can beseen in FIGS. 30 a and 30 b, the initial control curve shapes 3000extend straight along the normal from the surface, and the resultingcontrol hairs 3010 are shaped in order to match the controller curve3020.

In both modes, absolute and relative, the effect of the super hair isapplied uniformly to all the driven hairs. For localized control, aregion of influence can be defined around the controller curve.

Blend balls including inner and outer spheres (as previously-described)may be used to define where and by how much a super hair has an effect,as shown in FIG. 31. In particular, FIG. 31 is a diagram illustrating ablend ball 3110 having both an inner sphere 3112, and an outer sphere3114 and the blend balls 3110 effect upon the controller curve 3120 andcontrol hairs 3130.

Both an inner and outer weight are associated with each correspondingsphere 3112 and 3114 and the effect of the super hair 3120 and controlhairs 3130 is interpolated in-between. As a practical application of theregion of influence, the super hair 3120 can be used to fake collisionsuch as a hand rubbing over a part of fur by having the controller curveslide on the surface to follow the hand motion and squash the hairs downas it moves.

Exemplary Uses of Hair Motion Compositing

Discussed below are some examples of the power and versatility of thehair motion compositor (HMC).

Improving a Single Simulation Result

Visual effects productions tend to be heavily art-directed for both liveaction and computer graphic features. One of the most frequentcomponents from an initial hair shot review is to make a dynamicsimulation match the artistically approved look more closely. This mayentail either making the simulation stiffer, to remove extra or erraticmotion, or making it match the key framed goal animation in a betterfashion.

Oftentimes, the simulation may produce physically-realistic results, butnot necessarily the desired look. Rather than relying on a physicallyaccurate solver by trying to fine-tune dynamic parameters and launchingnew simulations over and over again, it is often easier to“composite-in” some of the static or goal animation using the blend nodefunctionality previously described with reference to FIGS. 24, 25, 26,27, and 31. As has been discussed, the blend factor can be set oranimated to preserve as much of the dynamic simulation that is found tobe visually pleasing.

Blending Between Multiple Simulation Results

As previously described, simulation results rarely provide the perfectdesired look “out of the box”. With varying sets of input parameters,one simulation might give a good starting motion, but another mightprovide a nicer end look, and a third might be more expressive atcertain key moments. Finding a unified set of parameters that wouldprovide a combination of all three is often impossible.

However, with the features of the hair motion compositor (HMC), aspreviously described, a cascading graph may be easily set up to blendbetween various simulation caches as illustrated in FIG. 32.

As can be seen in FIG. 32, three hair caches 3200, 3202, and 3204, areeach respectively processed through animation nodes 3210, 3212, and3214. Hair caches 3200 and 3202 are further combined through blend node3220 and further through animation node 3222 and further blended withhair cache 3204 at blend node 3230.

This blended output is then processed though animation node 3240 andblended at blend node 3245 with a static node 3248 and a final processoutput is rendered at control node 3250. It should be appreciated thatthe blend values can be key-framed to pick up various parts of eachcache that are most pleasing.

Blending can also be used for non-dynamic cache files. For characterswith short fur, it may be practical to have two static combs: one“normal” and one “collided” with the hairs closely pushed down towardsthe surface. Then, instead of simulating all the interactions with thefur in a shot such as hands or props rubbing against it, the hair cansimply be blended to the pre-collided comb in those areas, using blendballs that follow the collision objects.

In addition to its blending power, the hair motion compositor (HMC)system may be utilized to correct problems in particular shots. Forexample, this often occurs in one of two situations that often cause badresults: animation issues which make static combed goal hairs penetratethe characters skin, and minor simulation errors such as missedcollisions. In these cases, the general motion of the hair is perfectlyacceptable, and all that is required to correct the problems may beaccomplished by the use of volume offset nodes or super hair nodes,previously discussed. This is much faster than re-running a simulationuntil it is satisfactory.

Optimization Techniques

As previously discussed with reference to FIG. 15, embodiments of theinvention further relate to optimization techniques that may beimplemented by optimization module 1567 of hair/fur pipeline 1500. Inparticular, three particular optimization techniques that improve theusability and rendering speed of hair/fur pipeline 1500 are disclosedherein.

The first optimization technique relates to fine-grain control over theculling of final hair follicles dependent upon screen-space metrics.Referred to herein as view-dependent screen-space optimizationtechniques. A second optimization technique disclosed herein relates tothe hair/fur pipeline's 1500 ability to selectively generate and rendervisible hair follicles based upon hair sub-patches. Referred to hereinas hair sub-patch optimization techniques. Further, a third optimizationtechnique implemented by the hair/fur pipeline 1500 relates to haircaching in order to decrease render times and improve turnaround timefor lighting work. Referred to herein as hair caching techniques.

View-Dependent Screen-Space Optimization

As is well known, rendering a fully-furred creature at maximum hairdensity often takes a long time, utilizes a great deal of computingpower, and utilizes a significant amount of memory. Although it ispossible to manually adjust hair density in order to optimize hairdensity on a per-shot basis, this is a tedious and error-prone process.

According to one embodiment of the invention, view-dependentscreen-space optimization techniques implemented by the optimizationmodule 1567 of the hair/fur pipeline 1500 may be implemented byutilizing continuous metrics measured in screen-space that give bothfine-grain control over the setting of hair parameters—such as hairdensity and hair width parameters at render time. Further, the behaviorof optimization parameters may be customized by user-defined functioncurves.

In one embodiment, surface definition module 1550 may be used to definea surface and optimization module 1567 may be used to determine whethera hair is to be rendered upon the surface. In particular optimizationmodule 1567 may be used to: determine a size metric for the hair; applya first density curve to the size metric determined for the hair togenerate a density multiplier value; and based upon the densitymultiplier value, determine whether the hair should be rendered.

The continuous metrics and customizability of optimization parametersenables a great deal of flexibility in this method and does not sufferfrom constraints associated with discrete selection of pre-determinedvalues. In one embodiment of the present invention, the optimizationparameters, as will be discussed hereinafter, relate to hair density andhair width parameters. However, it should be appreciated that othertypes of hair-related parameters may also be optimized.

In this embodiment, hair screen-space optimization deals withdetermining the amount to reduce the number of rendered hairs based uponthe size of each individual hair on the screen (screen-space sizemetric) and on the speed at which the hair travels onscreen(screen-space speed metric).

Both metrics are calculated per hair and are then passed back to a userspecified function curve that specifies whether or not a hair should berendered. By using a custom function curve, a character's hair can becustomized on a per shot basis and thus a great deal of flexibility isgiven to the artist to determine how much hair to cull from a character.

With reference to FIG. 33, FIG. 33 is a diagram illustrating techniquesto implement view-dependent screen-space optimization. As can be seen inFIG. 33, view-dependent screen-space optimization 3300 is split into twostages. The first stage is a metric generation stage 3310 and the secondstage is a metric-to-parameter mapping stage 3320. In particular, eachhair follicle 3308 is fed into the metric generation stage 3310.

In particular, each hair follicle 3308 is fed into the metric generationstage 3310, and with additional reference to FIG. 34, each hair follicleroot position 3402 for each hair 3410 is transformed into a normalizeddevice coordinate (NDC) frame in the metric generation stage 3310 togenerate a proxy hair 3405 that replaces the original hair.

More particularly, in the metric generation stage 3310, the NDC rootposition 3402 of each hair is used to measure different metrics thatprovide additional information to decide how to control per-hairparameters at render time.

In this embodiment, two different metrics are utilized in the metricgeneration stage 3310—a size metric 3312 and a speed metric 3314.However, it should be appreciated that a wide variety of other differentmetrics may also be utilized. The screen-space size metric 3312 isutilized to calculate the length of a proxy hair 3405 in NDC space. Aproxy hair 3405 is a unit length hair that is grown straight up from thehair follicle root position 3402. A proxy hair 3405 is utilized insteadof an original final hair 3410 because it is easier to work withcomputationally than a final hair. This is because hair interpolationand effect operations associated with final hairs are verycomputationally intensive to perform especially if the hair iseventually culled. Another reason is that all hairs that are equaldistance from the camera, but differing in length, should be treatedidentically. By doing so, looks with large hair-length variations arepreserved, whereas ignoring proxy hairs in such a case may lead to baldspots or a different look once optimization is applied.

Screen-space speed metric 3314 calculates the distance traveled by theroot of a hair follicle from the current frame to the next frame in NDCspace. Referring briefly to FIG. 35, FIG. 35 is a diagram illustratingthe distance traveled by the root 3502 of a proxy hair 3506 from a firstframe 3510 at time t to a second frame 3520 at time t+1 in NDC space.This metric has been chosen because heavily motion-blurred objects oftendo not require full hair density to achieve the same look.

With reference back to FIG. 33, after the size metric 3312 and the speedmetric 3314 are determined at the metric generation stage 3310, thesevalues are passed to the metric-to-parameter mapping stage 3320. Foreach parameter, an operator applies the corresponding function curves toeach metric value. In this example, density curves 3324 and 3326 areapplied to the size metric 3312 and speed metric 3314 (e.g., density vs.size and speed), respectively, and width curves 3330 and 3332 areapplied to size metric 3312 and speed metric 3314 (e.g., width vs. sizeand speed), respectively. Each of the respective results are thenmultiplied to derive the final parameter multiplier value. In thisexample, a density multiplier value 3340 and a width multiplier value3350 are determined that can be passed onto rendering functionality3360.

It should be noted that in the case of the density multiplier, that theuser-defined function curves 3324 and 3326 are constrained to map metricvalues in the range from [0,1]. The final result, the density multipliervalue 3340, which also correspondingly varies between [0,1], is thenused to determine whether or not a final hair is rendered at renderblock 3360. In one example, this may be done by generating a randomnumber in the range [0,1]; if this number is lower than the finalresult, then the hair is drawn. Otherwise, it is culled.

For the width multiplier, the user-defined function curves 3330 and 3332are constrained to any non-negative real number and thus the finalresult is also a non-negative real number. This final result, widthmultiplier 3350, is then multiplied with the current hair's widthparameter and passed on to render block 3360.

It should be noted that because the hair count is no longer static, asan object (e.g., a character with hair) moves from the foreground to thebackground, that popping may occur. In order to alleviate this behavior,empirical testing has been done that shows that fading-out hairfollicles in conjunction with these optimization techniques minimizesthis effect. In view of this, each hair follicle may be forced to firstpass through a visibility determination and if it is determined to benon-visible, the hair's opacity may be reduced by linearly reducing thehair's opacity value.

It should be appreciated that by utilizing the above-defined techniquesthat the number of hairs generated or rendered may be reduced by thedensity multiplier value and the width of the remaining hairs may beincreased by the width multiplier value. Therefore, by utilizing thespeed and size metrics and the four function curves, these optimizationtechniques determine whether to cull a hair and by how much to scale itswidth.

In particular, utilizing these techniques for a fully furred creaturecan result in significant processing and memory savings. This is becauseif a creature is not close up on the screen or is moving quickly acrossthe screen, these techniques allow for the rendering of less than all ofthe hair that the creature had originally been designed with, withoutmuch of a visual difference, but render times and memory requirementsare improved significantly.

With reference to FIG. 36, FIG. 36 shows a table 3600 that presents aside-by-side comparison of non-optimized values in terms of hair count,time, and memory versus optimized values in terms of hair count, time,and memory using the screen-space size metric at various frames. Rendertime (“time”) is listed in minutes and seconds and memory usage (“mem”)is given in Megabytes. In this test, illustrated in table 3600, a furrycharacter was moved from a close-up position at frame 10 to far awayfrom a camera at frame 100. The function curves used in this examplewere aggressive because the actual hair count goes from 78% of theoriginal hair count at frame 10 to 1% at frame 100. As can be seen intable 3600, both render times and memory usage improve significantly.

It should be noted that the view-dependent screen-space optimizationtechniques are flexible in the sense that the function curves can easilybe adjusted to fine tune the optimization so that the character looksgood at any distance and the memory and render time savings are as highas possible.

With reference to FIG. 37, FIG. 37 is a table 3700 (similar to table3600) illustrating another comparison utilizing the screen-space speedmethod. In this example, the character is shot moving across the screenvery quickly and is rendered with motion blur. Frames were chosen wherescreen-space velocity is high. It should be noted that the hair countfor the character is reduced to 19%. This technique shows dramaticsavings as can be seen in table 3700 in both render time and memory forfast moving shots.

Further, tests have been performed with both size and speed metricsbeing utilized in conjunction with one another. In particular, withreference to FIG. 38, FIG. 38 is a table 3800 (similar to the othertables 3600 and 3700) that illustrates non-optimized and optimized haircount, time, and memory values. In this example, a character wasrendered with motion blur moving from far back at frame 101 to a muchcloser frame at frame 270. As can be seen in the values of table 3800both metrics work well with each other and save on rendering time andmemory and further have been shown not to sacrifice visual quality.

Hair Sub-Patch Optimization

Another optimization technique implemented by the optimization module1567 of hair/fur pipeline 1500 relates to hair sub-patch optimization.In general, this optimization is useful in rendering large amounts ofhair based upon the fact that not all of the hair is visible to thecamera at all times. Instead of generating all of the hairs and allowingdisplay module 1575 to perform visibility culling during rendering, hairpipeline 1500 utilizing optimization module 1567 can save rendering timeby not generating the non-visible primitives to begin with. Inparticular, aspects of these techniques may be implemented by theoptimization module 1567 in conjunction-with display module 1575.

In one embodiment, surface definition module 1550 may be used to definea surface and optimization module 1567 may be used to: create a boundingbox for a set of hairs; determine whether the bounding box is visible;and if the bounding box is visible, hair associated with the visiblebounding box is rendered upon the surface.

In general a bounding box may be created for the initial set of hairs tobe rendered and may then be tested for visibility before rendering. Ifthe bounding box is visible, the hair primitives are spatially dividedinto four sub-groups and a new bounding box is created for eachsub-group and each sub-group is again tested for visibility. If abounding box is determined to be not visible, then no furthersubdivision is performed on that hair group, nor is it rendered. Afterrecursively sub-dividing all the visible hair groups, a predeterminednumber of times, the hair primitives of each group are then sent todisplay module 1575 for rendering at that point.

With reference to FIG. 39, FIG. 39 is a flow diagram illustrating aprocess 3900 to implement hair sub-patch optimization. As can be seen inFIG. 39, at block 3910 a bounding box is created for an initial set ofhairs. Next, at block 3915, it is determined whether the bounding boxfor the initial set of hairs is visible. If not, process 3900 ends(block 3920). For example, the sub-groups may be spatially divided inapproximately equal relation. However, if the bounding box or a portionof the bounding box is visible for the initial set of hairs, then thebounding box is spatially divided into sub-groups (block 3925). Forexample, the sub-groups may be spatially divided in approximately equalrelation. Further, a bounding box is created for each sub-group (block3930).

Reference can also be made to FIG. 40 which is a diagram illustrating asimplified example of hair sub-patch optimization. As can be seen inFIG. 40, a first frame 4002 is shown with a bounding box 4010 for aninitial set of hairs to the right side of the frame. As can be seen, thebounding box 4010 for the initial set of hairs is visible such that itis visible and, as shown in frame 4004, bounding box 4010 is thenspatially divided into sub-groups or sub-group bounding boxes 4012.

Returning to process 3900 of FIG. 39, at block 3935 it is determinedwhether these sub-group bounding boxes are visible. If not, at block3940 the process ends. However, if these sub-group bounding boxes arevisible then it is determined whether a predetermined number ofsubdivisions have been met (block 3945). If not, the sub-group isfurther divided and is recursively subdivided a predetermined number oftimes. Assuming that the predetermined number has been met, then atblock 3950, the hair primitives of the sub-group bounding boxes arerendered.

Particularly, with reference to FIG. 40, as can be seen at frame 3 4006,the two leftmost sub-group bounding boxes 4014 are determined to bevisible (and are identified with hatch-marks) whereas the two rightmostboxes are not visible. Accordingly, the hair associated with the twoleftmost visible sub-group bounding boxes 4014 are sent on for renderingwhereas the hair associated with the two rightmost non-visible sub-groupbounding boxes are discarded. It should be appreciated that this can berecursively implemented a pre-determined number of times.

Further, because of the flexibility of the hair/fur pipeline 1500 aspreviously discussed with reference to the geometric instancingembodiment, environmental modelers may utilize aspects of the hairsub-patch optimization to extend it to other computer graphic uses suchas to render large grassy landscapes. By utilizing embodiments of thehair sub-patch optimization embodiment, from shot to shot, only a smallfraction of the entire landscape may need to be rendered resulting insubstantial processing and memory savings. However, it should beappreciated that by utilizing the previously-described geometricinstancing embodiments that any user-selected or randomly-generatedgeometric object may be rendered instead of hair with or withoutassociated hair parameters (bend, rotation, wind, etc.) and the axialdeformation parameters associated with the hair.

For example, to define a landscape, the process may be initiated with asingle bounding box that encompasses a patch defining the entirelandscape. This patch then may be subdivided into four disjointquadrants with smaller bounding boxes, as previously described. Forbounding boxes that get culled, no further processing is done, but forbounding boxes that continue to be processed, further subdivision willoccur until a user-defined stopping criterion is reached.

Additional stopping criterion may be utilized as well, such assubdivision depth and parametric limits relating to the parametriclength of the sub-patches: dimensions and parametric texture space.

Utilizing the previously described hair sub-patch optimizationtechniques discussed with reference to FIGS. 39 and 40 as applied to alandscape, a single large grassy landscape was modeled with 25 millionhairs in which the hairs were geometrically instanced with grass and/ortrees. But as shown in the frame of FIG. 41, only a few hundred bladesof grass were actually generated and rendered utilizing the hairsub-patch optimization techniques. Without this optimization, to renderall the grass would have resulted in a far greater amount of processingand memory usage being required.

Hair Caching Optimization

During the lighting phase, a lighting expert typically works on a shotthat has been approved by both the animation and layout departments.What is typically left to do is to determine the number and placement oflights and to create the type of lighting conditions needed to achievethe desired look.

In particular, during the lighting phase, all the elements of the scenehave been finalized including all of the hair parameters. Because ofthis, it is possible to cache out and re-use hair geometry and hairparameters in order to obtain significant processing and memory savings.In one embodiment, optimization module 1567 in conjunction with shadingand backlighting module 1570 may be utilized to implement techniquesaccording to the hair caching embodiment as described herein.

In order to accomplish this, a cache hair state file may be createdand/or validated and the state of the hair parameters, as they are to berendered, can be saved. For example, these hair parameters may be savedas an unordered list of hair parameters within the cache hair statefile.

In one embodiment, lighting module 1570 may be used to produce lightingeffects in a lighting phase for a shot and optimization module 1567 maybe used to: determine if a cache hair state file including hairparameters exists; and determine if the cache hair state file includesmatching hair parameters to be used in the shot, and if so, the hairparameter values from the cache hair state file are used in the lightingphase.

Turning to FIG. 42, an example of a cache hair state file 4200 thatincludes a list of hair state parameters 4202, 4204, 4206, etc., isillustrated. If the cache hair state file 4200 does not exist orcontains different parameter values than the state of the currentrender, then the new set of values may be saved into the cache hairstate file 4200 (i.e., as hair state parameters 4202, 4204, 4206, etc.).This new or updated cache hair state file 4200 can then be utilized.However, if the cache hair state file 4200 already matches the state ofcurrent render, then the original cache hair state file 4200 may beused.

Turning to FIG. 43, FIG. 43 is a flow diagram illustrating a process4300 to implement hair caching. At block 4302 it is determined whether acache hair state file 4200 exists. If not, a new cache hair state file4200 is created (block 4304). Then, at block 4306 the new cache hairstate file 4200 is saved.

However, if a cache state file does exist, then at block 4310 it isdetermined whether or not the hair cache state file 4200 includes thesame parameters to be utilized for rendering. If so, the process movesto block 4320 as will be described. Otherwise, at block 4306, thedifferent hair state parameters are saved to the cache hair state file4200.

With either a new cache hair state file, a cache hair state file withthe same parameters, or a cache hair state file with differentparameters, the process moves on to block 4320, where it is determinedwhether or not a proper file key is present. Each render and cachevalidation process is locked with an exclusive file lock prior to cachevalidation, cache generation and cache rendering. Accordingly, a properkey needs to be present for the cache hair state file to be used in thelighting phase. This is because multiple processes are often used togenerate a final rendered frame while a lighter is working on a shot.

If the proper file key is not present, then the process ends at block4340 and the cache hair state file 4200 is deleted. However, if thecache hair state file 4200 was properly unlocked, then the cache filemay be utilized in the lighting phase at block 4350.

In one embodiment, the physical cache hair state file may be representedas a RENDERMAN file. This may be done without a loss of generality sinceit simply encodes data points and parameters needed to generate hairgeometry. In-addition, compressed RENDERMAN files may be used as a spacesaving technique to represent cache files.

Thus, the previously described process 4300 determines whether or notthe cache hair state file 4200 is valid or invalid. The cache hair statefile is generally determined to be invalid if it does not exist or theprimitives found inside the cache file do not match the hair primitivesthat are to be rendered. In particular, to determine the lattercondition, each cache hair state file may be required to contain thehair render settings that were used to generate it in the first place.

Thus, a cache hair state file is deemed valid if the render settingsmatch. On the other hand, if the cache hair state file is found to beinvalid, then the correct hair parameters are fed and stored to cachehair state file, as previously described.

Typically, in the first pass, the cache is invalid and thus needs to begenerated (cache generation and use). Savings can be found here due toserialization that allows for immediate the use a cache once anotherprocessor has generated it. This alone cuts down rendering times byapproximately 69%. Further savings may be realized when differentprocesses are utilizing the same cache file. It should be noted thatlighters typically keep re-rendering many times and the actual timesavings are often multiplied.

Turning now to FIG. 44, FIG. 44 shows a table in which the time-savingsa lighter can accomplish by using the previously-described techniques torender a fully-furred character is illustrated. As can be seen in FIG.44, time to render without cache, time to generate and use cache forrendering, and time to render with the existing cache are shown. As canbe seen significant time-savings are realized.

In this description, numerous specific details are set forth. However,it is understood that embodiments of the invention may be practicedwithout these specific details. In other instances, well-known circuits,structures, software processes, and techniques have not been shown inorder not to obscure the understanding of this description.

Components of the various embodiments of the invention may beimplemented in hardware, software, firmware, microcode, or anycombination thereof. When implemented in software, firmware, ormicrocode, the elements of the embodiment of the present invention arethe program code or code segments to perform the necessary tasks. A codesegment may represent a procedure, a function, a subprogram, a program,a routine, a subroutine, a module, a software package, a class, or anycombination of instructions, data structures, or program statements. Acode segment may be coupled to another code segment or a hardwarecircuit by passing and/or receiving information, data, arguments,parameters, or memory contents. Information, arguments, parameters,data, etc. may be passed, forwarded, or transmitted via any suitablemeans including memory sharing, message passing, token passing, networktransmission, etc.

The program or code segments may be stored in a processor readablemedium or transmitted by a computer data signal embodied in a carrierwave, or a signal modulated by a carrier, over a transmission medium.The “processor readable or accessible medium” or “machine readable oraccessible medium” may include any medium that can store, transmit, ortransfer information. Examples of the machine accessible medium includean electronic circuit, a semiconductor memory device, a read only memory(ROM), a flash memory, an erasable ROM (EROM), a floppy diskette, acompact disk (CD-ROM), an optical disk, a hard disk, a fiber opticmedium, a radio frequency (RF) link, etc. The computer data signal mayinclude any signal that can propagate over a transmission medium such aselectronic network channels, optical fibers, air, electromagnetic, RFlinks, etc. The code segments may be downloaded via computer networkssuch as the Internet, Intranet, etc. The machine accessible medium maybe embodied in an article of manufacture. The machine accessible mediummay include data that, when accessed by a machine, cause the machine toperform the operation described in the following. The term “data” hererefers to any type of information that is encoded for machine-readablepurposes. Therefore, it may include program, code, data, file, etc.

More particularly, all or part of the embodiments of the invention maybe implemented by software. The software may have several modulescoupled to one another. A software module is coupled to another moduleto receive variables, parameters, arguments, pointers, etc. and/or togenerate or pass results, updated variables, pointers, etc. A softwaremodule may also be a software driver or interface to interact with theoperating system running on the platform. A software module may also bea hardware driver to configure, set up, initialize, send and receivedata to and from a hardware device.

While the invention has been described in terms of several embodiments,those of ordinary skill in the art will recognize that the invention isnot limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting.

1. A hair pipeline for generating hair comprising: a surface definitionmodule to define a surface; and an optimization module to: create abounding box for a set of hairs; determine whether the bounding box isvisible; and if the bounding box is visible, hair associated with thevisible bounding box is rendered upon the surface.
 2. The hair pipelineof claim 1, wherein, if the bounding box is not visible, the hairassociated with the bounding box is not rendered.
 3. The hair pipelineof claim 1, wherein the optimization module further: sub-divides thebounding box into sub-group bounding boxes; determines whether thesub-group bounding boxes are visible, and if so, hair associated withthe visible sub-group bounding boxes is rendered upon the surface. 4.The hair pipeline of claim 3, wherein, if a sub-group bounding box isnot visible, the hair associated with the sub-group bounding box is notrendered.
 5. The hair pipeline of claim 3, wherein the sub-groupbounding boxes are further sub-divided a pre-determined number of times.6. The hair pipeline of claim 3, wherein the bounding box and sub-groupbounding boxes are spatially divided in approximately equal relation. 7.The hair pipeline of claim 3 wherein a user-selected geometric object isrendered in place of a hair.
 8. A method to determine whether hair is tobe rendered comprising: creating a bounding box for a set of hairs;determining whether the bounding box is visible; and if the bounding boxis visible, rendering hair associated with the visible bounding box. 9.The method of claim 8, wherein, if the bounding box is not visible, thehair associated with the bounding box is not rendered.
 10. The method ofclaim 8, further comprising: sub-dividing the bounding box intosub-group bounding boxes; determining whether the sub-group boundingboxes are visible; and if the sub-group bounding boxes are visible,rendering hair associated with the visible sub-group bounding boxes. 11.The method of claim 10, wherein, if a sub-group bounding box is notvisible, the hair associated with the sub-group bounding box is notrendered.
 12. The method of claim 10, further comprising sub-dividingthe sub-group bounding boxes a pre-determined number of times.
 13. Themethod of claim 10, wherein the bounding box and sub-group boundingboxes are spatially divided in approximately equal relation.
 14. Themethod of claim 10, wherein a user-selected geometric object is renderedin place of a hair.
 15. A machine-readable medium containing executableinstructions which, when executed in a processing system, cause thesystem to perform a method to determine whether hair is to be renderedcomprising: creating a bounding box for a set of hairs; determiningwhether the bounding box is visible; and if the bounding box is visible,rendering hair associated with the visible bounding box.
 16. Themachine-readable medium of claim 15, wherein, if the bounding box is notvisible, the hair associated with the bounding box is not rendered. 17.The machine-readable medium of claim 15, further comprising:sub-dividing the bounding box into sub-group bounding boxes; determiningwhether the sub-group bounding boxes are visible; and if the sub-groupbounding boxes are visible, rendering hair associated with the visiblesub-group bounding boxes.
 18. The machine-readable medium of claim 17,wherein, if a sub-group bounding box is not visible, the hair associatedwith the sub-group bounding box is not rendered.
 19. Themachine-readable medium of claim 17, further comprising sub-dividing thesub-group bounding boxes a pre-determined number of times.
 20. Themachine-readable medium of claim 17, wherein the bounding box andsub-group bounding boxes are spatially divided in approximately equalrelation.
 21. The machine-readable medium of claim 17, wherein auser-selected geometric object is rendered in place of a hair.